This invention relates to electronic ballasts for gas discharge lamps and, in particular, to a low cost electronic ballast having high power factor and low harmonic distortion.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. An inductor in series with the lamp is a better ballast than a resistor in terms of efficiency but is worse in terms of power factor. Power factor is a term used in connection with load devices powered by alternating current (AC) to indicate whether or not the load is a pure resistance (power factor of 1.0). If a load has a power factor less than 1.0, the current and voltage in the circuit are out of phase, causing increased power consumption. Regulations in many states require a high minimum power factor, e.g. 0.85, as part of energy conservation measures and the effect of these regulations is to require the use of electronic ballasts.
An electronic ballast typically includes a converter for changing the AC from a power line to direct current (DC) and an inverter for changing the DC to high frequency AC. Converting from AC to DC is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. The voltage on the capacitor is not constant but has a 120 hz "ripple" that is more or less pronounced depending on the size of the capacitor and the amount of current drawn from the converter. Some ballasts include a "boost" circuit to increase the voltage on the capacitor from approximately 140 volts to 300 volts or higher. The inverter changes the DC to high frequency AC at 140-300 volts for powering one or more discharge lamps.
Electronic ballasts do not inherently have a high power factor. On the contrary, most electronic ballasts draw more current at low input voltage than at higher input voltage, just the opposite of a resistive load. A number of techniques have been proposed in the prior art for improving the power factor of electronic ballasts. One technique is to vary the power from the inverter to the lamp in phase with the ripple voltage. Another technique is to feed power from the inverter back to the converter to reduce current drain during low voltage portions of the input voltage. These techniques can improve power factor but do not improve harmonic distortion.
Harmonic distortion is a characteristic AC signals and relates to the harmonic content of a signal. As originally shown by the French mathematician Fourier, a square wave of a given frequency can be approximated by combining the fundamental and odd harmonics of a sinusoidal wave having the same frequency as the square wave. The waveform of the AC signal in a power line is sinusoidal and has a frequency of 60 hz. in the U.S.A. If the current drawn by a ballast is essentially in the form of square wave pulses, then the ballast is said to exhibit harmonic distortion because of the odd harmonics of 60 hz. in the current through the ballast.
One can improve the power factor of a ballast and still exhibit a large harmonic distortion. For example, a 60 hz. square wave signal can appear to have a power factor as high as 0.95. However, harmonic distortion is 30% or more. Harmonic distortion is a problem for the three-phase circuits typically found in commercial buildings because odd harmonics do not cancel out on the neutral line of a three phase system. Since the odd harmonics do not cancel out, power consumption is increased.
Electronic ballasts are intended to be connected to a load. If a lamp is not connected to the inverter or if a lamp is defective, the voltage on the connectors for the lamp can become even higher than the 300 volts available from the converter. This creates a hazardous situation for anyone who may come in contact with the ballast. Most ballasts have sensing circuitry for detecting a missing or defective lamp and for shutting off the inverter for a predetermined period of time. There is a problem in that the converter is still operating, producing in excess of 300 volts within the ballast.
In view of the foregoing, it is therefore an object of the invention to provide a low cost electronic ballast having increased power factor and reduced harmonic distortion.
Another object of the invention is to improve the power factor and harmonic distortion of an electronic ballast by varying the current drain of a boost circuit in accordance with the ripple voltage.
A further object of the invention to provide an electronic ballast in which both the converter and the inverter are shut off when a lamp is missing or defective.